It is known in oilfield drilling technology to provide an assembly, at the lower end of a drill string drilling a wellbore, for transmitting downhole data, measured by sensors, to a receiver at ground surface. This transmission is done by means of electric signals passing upwardly through the formation penetrated by the wellbore.
For example, it is known to measure the inclination and direction of the wellbore adjacent the bit using an EMT tool. The EMT tool includes a signal generating unit spaced behind the drill bit. This unit produces alternating current signals indicative of the measurements taken by downhole sensors.
In conjunction with the signal generating unit it is necessary to provide, in the drill string, a device that will block current flow through the conductive steel pipe of the drill string, to thereby induce the current flow to move upwardly to ground surface through the formations penetrated by the wellbore.
One known device in use for this purpose is commonly referred to as a ‘gap sub assembly’.
The gap sub assembly comprises a pair of tubular pipe subs threadably connected end to end by a pin and box connection. A ‘sub’ is industry terminology for a short length of pipe having threaded pin and box ends. A pin and box are threaded together to form a ‘connection’ joining the two subs end to end. The sub also usually has a shoulder, at the pin root, forming an annular face. In such a case, the box used forms an annular end face. These faces are provided to press together and seal when the connection is ‘made up’, thereby preventing the egress of drilling fluid into the well annulus.
Heretofore, commercially used gap sub assemblies have usually involved applying a coating of dielectric material, such as ceramic or the material identified by the trade-mark TEFLON™, and bonding it onto the threads of the pin and the end faces of the connection. In this way an electrically insulated gap is provided. When the gap sub assembly is incorporated into the electrically conductive drill string, the gap serves to block current flow along the string.
However, these known gap sub assemblies have shortcomings. The assemblies are subject to severe compressive and tensile stresses in use. The brittle ceramic coatings tend to crack and chip, thereby leading to loss of the electric discontinuity. And the Teflon™ coating may extrude during use, leading to the same result.
There therefore exists a need for a durable and relatively inexpensive gap sub assembly.